net: mtk_eth_soc: move restoration of SYSCFG0 to mac_finish()

The SGMIISYS configuration is performed while ETHSYS_SYSCFG0 is in a
disabled state. In order to preserve this when we switch to phylink_pcs
we need to move the restoration of this register to the mac_finish()
callback.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>